The present invention constitutes an improvement over the pile joint disclosed in Svensson British Specification No. 1,393,998 published Aug. 21, 1974. The Svensson joint consists basically of a pair of square base plates with corner slots. Anchor bars fixed to the base plates are used to attach the plates to the associated reinforced concrete pile sections. Locking pins serve to couple adjoining base plates and are double headed and circular in cross-section with the center stem being coaxial with the enlarged heads. A frusto-conical portion of the head is located at its junction with the center stem. The pile joint is made by driving the pins into the aligned slots at the corners of the coupled square plates.
A joint of the Svensson type presents a number of serious drawbacks. In this connection, the slots must be formed accurately at their inner ends in order to receive the circular stem of the pin. Bearing between pin and plate is on the tapered (frusto-conical) section of the pin head and over no more than 75% of the circumference of the pin head, the remainder being over the slot. Accordingly, contact area is small and high local stresses can occur in the metal of the pin. Thus, when the pile is in tension or bending, bearing stresses will be very high and deformations correspondingly pronounced. The bearing area can be increased only by decreasing the taper and increasing head diameter or increasing the whole pin diameter. This is self-defeating as it increases pin to anchor distance. Also, use of high tensile steel is expensive and leads to possible corrosion by electrolysis. None of these approaches offer a practical solution to this problem.
In the Svensson joint, although the head of the pin approaches the anchor bar attaching the joint plate to the reinforced concrete, the pin is nevertheless spaced from the axis of the anchor bar to a relatively significant extent. The distance between the end of the pin slot and the anchor bar is critical in the performance of the joint in tension or in bending. An increase in this distance dramatically decreases performance. In this connection, in order to transfer the forces from the reinforcement in one section of a pile to the reinforcement in the section of the pile which is being joined to the first section, it is of the greatest importance that the pin be located as close to the line of the reinforcement bars as possible. With the Svensson joint it is difficult to locate the stem of the pin close to the line of the reinforcement because of the pin shape. Accordingly, the distortion factor of the plate increases because as a practical matter, the driving forces are rarely truly axial thereby generating bending moments during the driving operation. In addition, lateral forces on buildings and structures supported by the piles similarly induce bending moments which have their effect on base joint distortion problems. Thus, strength can be regained only by an increase in basic plate thickness or decrease in head diameter of the pin. However, an increase in plate thickness requires increase in pin stem length and consequently increase in deformation of the joint in tension or bending and a decrease in head diameter of the pin would further reduce the contact area and increase the stresses in the pin head. Obviously these proposals do not offer satisfactory solutions to the deformation problem. Be that as it may, the walls of the slot recess of the Svensson joint prevents the pin from being located in close alignment with the reinforcement bars.
As indicated, the Svensson pin is circular with a small taper appearing at the junction of the head with the center stem. With a pin of this type, it is extremely difficult when making a pile joint to drive home the pin because its shape promotes tilting and above all the shank cannot be struck with a hammer or other impact imposing tool. Towards this end, it is necessary to strike both the top and bottom heads simultaneously with the hammer, otherwise tilting will occur. When the pin has been partially driven home, it is necessary to strike the cylindrical stem. This tends to rotate the pin and delay the operation. In addition, hammering on the relatively slender pin, increases the risk of deformation, damage or breakage. Obviously, at the pile driving site, in outdoor conditions, it is essential for cost savings that the pin driving process be as speedy as possible. The driving home of the pin requires a blow applied uniformly over the pin length, otherwise it jams without going fully home, thereby reducing joint efficiency.
Furthermore, in order to receive the cylindrical stem of the Sevensson pin, the inner part of the corner recesses must be accurately formed. This is a relatively expensive process in the manufacture of the plate. Above all, the cost of manufacturing the entire joint is proportionately increased.
Under field conditions, pile splices often times fail directly in the joint. To develop splice systems with strengths equal to or greater than the piles themselves is usually extremely costly.
Joints have also been proposed that involve welding. However, these are relatively time consuming during fabrication involving costly down-time on the driving rig. They also carry the inherent risk of damaging the adjacent concrete as a result of heat input.
Joints requiring bolts carry the risk of bolt lossening as a result of vibration during driving. Sleeved type joints have a very poor resistance to bending as well as tensile forces. Joints utilizing mechanical locking are generally preferable, providing there is no risk of the mechanical connection being loosened.
Inherent in many of the prior art joints is the risk of damage when the section is being handled and when it is being driven. If a machined groove or face of projection is damaged, the matching component will not fit. The consequences of this are either (a) delays while difficult repairs are being carried out; (b) abandonment of piles; (c) continuation of driving while connection is imperfectly made.